Pulsed fluidised bed

Abstract

An apparatus and method for processing materials in a batchwise or continuous fluidised bed, such as a drier, in which the fluidised bed is subdivided into a plurality of smaller areas, to each of which two separately controlled gas flows are provided. The first lower gas flow is provided to the bed preferably all of the time, and is sufficient at least to maintain the bed in an expanded state. The second higher gas flow is provided to each separate area of the bed in sequence by means of a rotary valve arrangement, and is high enough to induce fluidisation in the bed, but not high enough to induce significant solids loss by entrainment in the offtake gas. The higher flow sequence can be chosen to provide almost any desired sequence to the separate parts of the bed. The sequence can be chosen to induce a travelling wave within the bed which can be across or along the bed, and can be skewed relative to the sides of the bed. In a continuously operated bed, the travelling wave can move either co-currently with, counter-currently to, or cross-currently to, the direction of flow of the solids through the bed. This form of operation, particularly with a skewed travelling wave, improves bed operating efficiency. Since the rotary valve cannot be closed completely, problems associated with abrasive dust in the gas feed, of valve lubrication, and valve differential thermal expansion are all substantially avoided.

What we claim is: 
     
       1. An apparatus for feeding gas to a pulsed fluidised bed contained within a bed chamber having walls to contain the bed and a grid beneath the bed, comprising in combination: (i) at least one gas plenum chamber having outer walls disposed beneath and in a sealed relationship with the walls of the chamber;   (ii) internal walls within each gas plenum chamber which extend to the underside of the grid and divide the gas plenum chamber into a plurality of gas boxes;   (iii) at least one gas feed chamber attached to a wall of each gas plenum chamber;   (iv) a valve means in the gas path between each gas feed chamber and each plenum chamber including a rotating disc valve having at least one valve port and a stationary valve seat having flow apertures which provide a gas flow path between the gas feed chamber and each gas box within the plenum chamber as the valve rotates; and   (v) a valve rotation means;   wherein: (a) the disc valve is separated from the valve seat and rotates in a plane substantially parallel to but spaced from the seat;   (b) the disc valve port has an effective diameter D;   (c) the stationary valve seat has an effective diameter of at least D; and   (d) the valve means includes at least one bypass port in addition to the disc valve port providing a gas flow to all of the gas boxes sufficient to maintain the bed in an expanded state, and to substantially prevent solids loss from the bed.     
     
     
       2. An apparatus according to claim 1 wherein: (a) the disc valve is separated from the valve seat by an axial distance b, and rotates in a plane substantially parallel to but spaced from the seat;   (b) the disc valve port has an effective diameter D;   (c) the stationary valve seat has an effective diameter of at least D; and   (d) the maximum value of b is 0.2 D, and the minimum value of b provides a gas flow sufficient to maintain the bed in an expanded state, and to substantially prevent solids loss from the bed.   
     
     
       3. An apparatus according to claim 1 wherein: (a) the disc valve rotates in a plane substantially parallel to but spaced from the seat;   (b) the disc valve port has an effective diameter D;   (c) the stationary valve seat has an effective diameter greater than D; and   (d) the disc valve peripheral diameter is smaller than the diameter D of the stationary valve seat by an amount b' so as to provide an annular by pass port around the periphery of the disc valve which provides a gas flow sufficient to maintain the bed in an expanded state, and to substantially prevent solids loss from the bed.   
     
     
       4. An apparatus according to claim 1 wherein: (a) the disc valve is separated from the valve seat by an axial distance b, and/or a radial distance b', and rotates in a plane substantially parallel and close to but spaced apart from the seat;   (b) the disc valve port has an effective diameter D;   (c) the disc valve further includes at least one subsidiary port of area b";   (d) the stationary valve seat has an effective diameter of at least D; and   (e) the separation distance b and/or b' between the disc valve and the valve seat, and the area b" of the subsidiary ports combined provide a bypass port which provides a gas flow sufficient to maintain the bed in an expanded state, and to substantially prevent solids loss from the bed.   
     
     
       5. An apparatus according to claim 1 wherein the, or each, disc valve has one port. 
     
     
       6. An apparatus according to claim 2 wherein the, or each, disc valve has an effective diameter D, a single port of area A, and b is about A/πD. 
     
     
       7. An apparatus according to claim 3 wherein the, or each, disc valve has an effective diameter D, a single port of area A, and b' is about A/πD. 
     
     
       8. An apparatus according to claim 4 wherein the, or each, disc valve has an effective diameter D, a single port of area A, and b, b' and/or b" separately or in combination as appropriate provide a bypass port or ports having a total area corresponding to a separation between the disc valve and the valve seat of about A/πD. 
     
     
       9. An apparatus according to claim 1 including one plenum chamber. 
     
     
       10. An apparatus according to claim 1 including at least two plenum chambers with separate rotary valve means in each plenum chamber. 
     
     
       11. An apparatus according to claim 10 including at least two plenum chambers separated by a longitudinal common wall with separate rotary valve means in each plenum chamber. 
     
     
       12. An apparatus according to claim 11 wherein the common wall includes perforations. 
     
     
       13. An apparatus according to claim 12 wherein the perforations provide an open area of less than about 5%. 
     
     
       14. An apparatus according to claim 12 wherein the perforations provide an open area of about 3%. 
     
     
       15. An apparatus according to claim 11 wherein the rotary valve ports are displaced relative to each other by a phase angle γ having a value between zero and 180°. 
     
     
       16. An apparatus according to claim 15 wherein the rotary valve means are substantially coaxial, and the phase angle γ is 360°÷n, in which n is the number of gas boxes to which gas is fed during one complete rotation of the valve. 
     
     
       17. An apparatus according to claim 1 wherein the valve rotation means provides a frequency of pulsation of each area of the fluidised bed of from about 1 Hz to about 50 Hz. 
     
     
       18. An apparatus according to claim 1 wherein the valve rotation means provides a frequency of pulsation of each area of the fluidised bed of from about 4 Hz to about 15 Hz. 
     
     
       19. An apparatus according to claim 1 wherein the valve rotation means provides a frequency of pulsation of each area of the fluidised bed of about 10 Hz. 
     
     
       20. A method of pulsed fluidising a fluidised bed which comprises: (i) providing an independently controlled gas flow to different separate areas of the bed;   (ii) controlling the gas flow to provide to each area of the bed a lower gas flow level sufficient to maintain the fluidised bed in an expanded state, and sufficient to substantially prevent solids loss from the bed; and   (iii) further controlling the gas flow to provide separately, in a repeating predetermined sequence and at a predetermined frequency to each area of the bed a higher gas flow sufficient to fluidise the bed and insufficient to cause solids loss by entrainment in the gas flow from the bed.   
     
     
       21. A method according to claim 20 wherein the separate areas of the bed extend either transversely across, or longitudinally along the bed. 
     
     
       22. A method according to claim 21 wherein the higher gas flow is provided in sequence to each area of the bed either along or across the bed. 
     
     
       23. A method according to claim 22 wherein the higher gas flow induces a travelling wave in the fluidised solids of the bed. 
     
     
       24. A method according to claim 22 wherein the higher gas flow induces a skewed travelling wave in the fluidised solids of the bed. 
     
     
       25. A method according to claim 20 wherein the frequency is from about 1 Hz to about 50 Hz. 
     
     
       26. A method according to claim 20 wherein the frequency is from about 4 Hz to about 15 Hz. 
     
     
       27. A method according to claim 20 wherein the frequency is about 10 Hz.